The invention concerns a flat, multi-layer arrangement of superconducting wires with a normally conducting substrate and at least one fiber which is electrically conductively connected to the substrate and is superconducting at predetermined operating conditions.
A flat electrically conductive arrangement of this type is disclosed e.g. in U.S. Pat. No. 5,701,744.
Strong magnetic fields are used for nuclear magnetic resonance (NMR) spectroscopy or in magnetic resonance imaging (MRI) methods. These magnetic fields are generated using superconducting magnet coil systems. Optimum measuring conditions can be obtained, in particular, when the working volume inside the magnet coil system is not influenced by temporary external magnetic field fluctuations (stray fields).
Such stray fields may be caused, in particular, by cooling systems which cool the superconducting magnet coils. Widely used cooling systems are operated with periodically running adiabatic expansion of a working gas, such as helium, wherein a stray field of the same period is generated.
The stray field is generated e.g. by a movable ferromagnetic piston or by a magnetic phase change in a regenerator material. The cooling systems have relatively long cycle times in the range of one second (corresponding to a low working frequency of approximately 1 Hz).
To reduce the influence of stray fields on the working volume, an additional periodic field is conventionally generated using magnet coils to actively compensate for corresponding periodic stray fields in the working volume. It is, however, very difficult to generate an additional field which exactly compensates for a disturbance.
Inductive shielding of the working volume using a copper cylinder is also a conventional solution. The copper shielding operates efficiently at high disturbing frequencies but is insufficient at low disturbing frequencies, in particular, around 1 Hz.
In another conventional means, stray fields are neutralized using: a superconducting shielding collar which is locally disposed around the source of disturbance (see U.S. Pat. No. 5,701,744). Towards this end, a flat superconductor is inserted into the collar. These flat superconductors tend to be instable, since the current paths are not unambiguously defined. For small temperature fluctuations, the superconduction may temporarily fail thereby generating strong magnetic field disturbances in the working volume.
U.S. Pat. No. 5,701,744, which will be described in more detail below, discloses a main field magnet of a magnetic resonance apparatus comprising a superconducting main field coil for generating a temporally and locally constant magnetic field in a working volume. The main field coil is located in a liquid helium cryostat bath. The consumption of expensive liquid helium is prevented through use of a refrigerator having a cold head which is mounted to the cryostat. The cold head described in U.S. Pat. No. 5,701,744 has excellent cooling properties, since a so-called regenerator material of Er and Ni or Ho and Cu is used in its cold region, which is magnetically phase-changed at the low operating temperatures in the region of 3 to 20 K. The material, which is generally ferrimagnetic at low temperatures, is magnetized in the background field of the main field magnet. This causes an associated problem, since the magnetized regenerator material of the type of cold heads described in U.S. Pat. No. 5,701,744 periodically reciprocates according to the Gifford McMahon principle, thereby representing a temporally varying magnetic source of disturbance for the magnetic field in the working volume. In modern pulse tube coolers as are produced e.g. by the company Cryomech Inc. Syracuse, N.Y., USA, this problem is already considerably reduced since the regenerator material is no longer moved. The regenerator material thereby still represents a temporally periodic magnetic source of disturbance of reduced strength, since the temperature of the regenerator material and thereby of the magnetized volume portion periodically changes during operation. This is still a problem for magnetic resonance apparatus with particular quality requirements. The frequency of the disturbances caused by periodically operating refrigerators is between 0.1 Hz and 10 Hz.
U.S. Pat. No. 5,701,744 attempts to solve these problems by surrounding the so-called cold finger of the cold head containing the regenerator material, with a collar of a superconducting bismuth alloy, preferably lead-bismuth. In such a collar of ideal superconducting material, shielding currents are generated which largely compensate for the effect of the magnetic stray fields generated by the enclosed regenerator material outside of the collar where the investigational volume of the magnetic resonance apparatus is located. In view of the dimensions of the cold fingers of conventional coolers, such a collar has a diameter of at least approximately 0.05 m and a length of at least 0.1 m.
Through the excitation of corresponding shielding currents, such a collar of superconducting material shields both magnetic stray fields generated by the enclosed regenerator material towards the outside as well as the inner space surrounded by the collar from the static magnetic field of the main field magnet. This causes problems, since the superconducting material of the collar is necessarily flat and has transverse dimensions on the order of magnitude of 0.1 m. Moreover, the superconductors already tend to be unstable at transverse dimensions of less than 1 mm, which can lead to a sudden transition into the normally conducting state and an associated sudden drop in the shielding currents. This produces particularly strong disturbances of the magnetic field in the working volume of the NMR apparatus of a kind which should actually be prevented.
In contrast thereto, it is the object of the present invention to present a shielding for the working volume of a magnet coil arrangement which can efficiently and reliably shield low-frequency stray fields.